California’s urban heat islands are exacerbated by climate change in a manner that can compromise electricity sector resilience and public health. It is important to understand the factors that contribute to UHI, as well as how UHI can be mitigated. In this study, the research team carried out a multidimensional assessment of urban temperature variations based on state-of-the-science numerical modeling and several types of observations, including mobile transects and personal weather stations. The research team sought to understand spatial air- temperature variations in local heat and cool islands and the relationship to land-use and land- cover properties in the Los Angeles Basin.
The research team collected detailed LULC datasets for the Los Angeles Basin that were used as inputs for the meteorological modeling and analysis. The research team also developed a database of historical micrometeorological information from existing weather station networks. The research team collaborated with the City of Los Angeles, County of Los Angeles, and Los Angeles Unified School District to find sites and hosts for stationary monitors in two study areas that the research team identified, San Fernando Valley (SFV5) and downtown Los Angeles (LA1). The research team used fine-resolution climate models to assess the potential local UHI or UCI signals or both in these two areas. The results of the modeling informed the design of mobile-transect routes and siting of the stationary monitors. The research team designed a highly accurate and affordable stationary monitor that was installed in three locations in the SFV5 study area and at the University of Southern California campus west of downtown Los Angeles. To supplement the data collected from the stationary monitors, the research team completed 15 mobile transects. The research team designed and constructed an apparatus that is easily mounted on the roof of a car to measure air temperature along the transect routes. The research team assessed data from personal weather stations in the Weather Underground network to relate observed intraurban temperature variations to LULC and surface physical properties. The research team also analyzed observational micrometeorological data from mobile transects and correlated them with neighborhood-scale albedo and vegetation canopy cover.
The research team found the first observational evidence from analysis of high-spatial-density weather stations that increases in roof albedo at neighborhood scale are associated with reductions in near-surface air temperature. This evidence was corroborated with the analysis from mobile transect measurements, which revealed a cooling effect from areawide increase in albedo or canopy cover or both. The albedo and canopy cover values in this analysis are existing conditions in Los Angeles neighborhoods where there has not been a concerted effort to address UHIs. This finding suggests that there are opportunities for community cooling using existing technologies and practices to increase urban albedo and canopy cover. Knowing
that these are effective cooling measures, cities can also accelerate implementation of these UHI countermeasures in heat-vulnerable neighborhoods.
In addition, the calibrated meteorological model accurately identified the localized urban heat islands and urban cool islands observed in this study. Therefore, the model can be applied by stakeholders, including city and state government, to characterize the intraurban microclimate variations elsewhere in California, and stakeholders can use the model to analyze the benefits from launching UHI countermeasures.
30
GLOSSARY AND ACRONYMS
Term Definition
AGL Above ground level
Albedo Solar reflectance
Anemometer Wind speed meter
Aspirated radiation shield
Shield through which air is drawn to improve accuracy of air temperature measurement by an enclosed thermometer
CART Classification and regression tree: a multiple-regression statistical analysis often represented graphically with a tree-like structure Central Los
Angeles
A study region in Central Los Angeles used in the research team’s analysis of personal weather station measurements
Climate archipelago
The complex climate effects of a large, continuous urban area in which upwind portions can affect downwind portions
Cool roof
Roof whose surface stays cool in the sun by exhibiting high solar
reflectance (ability to reflect sunlight) and high thermal emittance (ability to emit thermal radiation)
Dry-bulb air
temperature Air temperature measured by a dry thermometer EPIC (Electric
Program Investment Charge)
The Electric Program Investment Charge, created by the California Public Utilities Commission in December 2011, supports investments in clean energy technologies that benefit electricity ratepayers of Pacific Gas and Electric Company, Southern California Edison Company, and San Diego Gas & Electric Company.
GPS Global positioning system
LA1 Study region near downtown Los Angeles used in the team’s climate- modeling analysis
LAUSD Los Angeles Unified School District
LDT Local daylight time
LULC Land use/land cover
MAE
Mean absolute error. This is a measure of the average deviation of a model prediction of a property, such as temperature, from the observed value of the property at the same location, time, and overall conditions.
Mesonet A regional network of weather stations
Meso-urban Scales that range from the regional to the sub-urban Mobile transect Measurement of temperature along a path
Monitor A device that measures and records weather parameters
PWS Personal weather station (a privately owned, consumer-grade stationary monitor)
Pyranometer Sunlight meter Roughness
length parameter
An indicator of the ability of a surface to exchange heat and momentum with the atmosphere
RSME
Root mean square error: the root of the mean of the squares of the errors (see definition of MAE above). RMSE is more sensitive to outliers than MAE.
SFV San Fernando Valley
SFV_R A study region in the San Fernando Valley used in the team’s analysis of personal weather station measurements
SFV5 Selected study region within the San Fernando Valley used in the team’s climate-modeling analysis
TSM Transect-specific (WRF) modeling: Weather simulations of the exact routes and at the exact times of the mobile transects
Thermistor Semiconductor thermometer
UCI Urban cool island
UHI Urban heat island
UHIE Urban heat island effect
Urban climate archipelago
The climate of an expansive continuous urban area or chains of urban areas that is large enough to produce its own characteristics and influences on the climate system
USC University of Southern California
WRF Weather Research and Forecasting (a numerical weather prediction system)
32
REFERENCES
Mohegh A., R. Levinson, H. Taha, H. Gilbert, J. Zhang, Y. Li, T. Tang, and G. Ban-Weiss. (2018). “Observational Evidence of Neighborhood-Scale Reductions in Air Temperature Associated With Increases in Roof Albedo.” Climate, 6, 98 (19 pp).
https://doi.org/10.3390/cli6040098
Taha, H., G. Ban-Weiss, S. Chen, H. Gilbert., H. Goudey, J. Ko, A. Mohegh, A., A. Rodriguez, J. Slack, T. Tang, and R. Levinson. (2018a). Modeling and Observations to Detect
Neighborhood-Scale Heat Islands and Inform Effective Countermeasures in Los Angeles.
Publication CCCA4-CEC-2018-007, California’s Fourth Climate Change Assessment, California Energy Commission.
http://climateassessment.ca.gov/techreports/docs/20180827-Energy_CCCA4-CEC-2018- 007.pdf
Taha, H., R. Levinson, A. Mohegh, H. Gilbert, G. Ban-Weiss, and S. Chen. (2018b). “Air-
Temperature Response to Neighborhood-Scale Variations in Albedo and Canopy Cover in the Real World: Fine-Resolution Meteorological Modeling and Mobile Temperature Observations in the Los Angeles Climate Archipelago.” Climate, 6(2), 53.
APPENDICES
Appendix A-1: Land-Use/Land-Cover Analysis and Atmospheric Modeling to Support Site
Identification and Selection for Fixed Meteorological Monitors and Mobile-Observation Routes
(Task 2 Report, Part I). Publication number CEC-500-2019-020-APA1.
Appendix A-2: Land-Use/Land-Cover Analysis and Atmospheric Modeling to Support Site
Identification and Selection for Fixed Meteorological Monitors and Mobile-Observation Routes
(Task Report Appendix A). Publication number CEC-500-2019-020-APA2.
Appendix B: Preliminary Mobile Measurements of Air Temperature via Transect (Task 2 Report, Part II). Publication number CEC-500-2019-020-APB.
Appendix C: Monitoring Plan, Instrumentation Design, and Sensor Deployment (Task 3 Report, Part I). Publication number CEC-500-2019-020-APC.
Appendix D: Atmospheric Modeling to Support Site Identification and Selection for Fixed Weather
Stations: Siting of Weather Stations (Task 3 Report, Part II) . Publication number CEC-500-2019-
020-APD.
Appendix E: Model Validation and Development of Correlations Between Observed Air
Temperature and Surface Physical Properties (Task 4 Report, Part I) . Publication number CEC-
500-2019-020-APE.
Appendix F: Air-Temperature Response to Neighborhood-Scale Variations in Albedo and Canopy
Cover in the Real World: Fine-Resolution Meteorological Modeling and Mobile Temperature Observations in the Los Angeles Climate Archipelago (Task 4 Report, Part II) . Publication
number CEC-500-2019-020-APF.
Appendix G: Observational Evidence of Neighborhood-Scale Reductions in Air Temperature
Associated With Increases in Roof Albedo (Task 4 Report, Part III) . Publication number CEC-500-